By: 1 September 2008


Obstetric ultrasonography has grown in popularity fast over the last decade and is now a standard component of antenatal care. Its safety and non-invasive nature make it acceptable to almost all, and it is used as a modern diagnostic and monitoring aid . It is also helpful as a tool to help take timely decisions and a guide for appropriate clinical interventions to reduce maternal and perinatal morbidity and mortality. The routine use of ultrasonography in the first and second trimester for screening for fetal anomalies is well-established and is a common obstetric practise worldwide.However its use on the labour ward has still not been fully appreciated although the indications for its use are growing rapidly. The use of ultrasound on labour ward was recently studied prospectively in a 4 month period in the U.S involving 2679 admissions to the labour ward, including 1363 non-labouring patients1. Almost 1 in 4 women admitted to labour ward required ultrasound scanning. Of those, approximately 68% were performed in non-labouring patients. The indications for ultrasound scanning in non-labouring women were mainly: amniotic fluid volume index in 15.8%, SROM in 15.6%, postdate in 9.8%, placental location in 9.6% and decreased fetal movement in 9.3%. The indications in labouring women were fetal presentation in 34.4%, confirmation of vertex presentation in 20.3%, preterm labour in 12%, multiple gestations in 7.3% and malpresentation in 7.3%. Overall, ultrasound scanning was performed in 15% of women in labour and 31% of non-labouring women. This study highlighted the importance of ultrasound as a diagnostic tool in the labour ward setting.

Ultrasound scanning on labour ward is used for various clinical indications as listed below:

  1. Preterm labour
  2. Unbooked pregnancies
  3. Preterm Premature Rupture of Membranes (PPROM), Spontaneous Rupture of Membranes (SROM)
  4. External Cephalic Version
  5. Fetal well-being
  6. Fetal viability
  7. Antepartum haemorrhage
  8. Multiple pregnancies
  9. Induction of labour
  10. Labour and Instrumental Delivery
  11. Prior to Caesarean section
  12. Post-partum period
  13. Anaesthesia related procedures
  14. Research

Preterm Labour
Spontaneous preterm delivery occurs before 37 weeks gestation in about 10% of pregnancies and is a major cause of perinatal morbidity and mortality. Diagnosis of preterm labour is crucial as it indicates the need to take further measures to improve the neonatal outcome such as hospitalisation, administration of steroid injections, use of tocolytics and in-utero transfer, if appropriate. However only a minority of pregnant women admitted with threatened preterm labour with regular contractions2, deliver before term.

A short cervical length and a raised cervical-vaginal fetal fibronectin concentration are shown to be the strongest predictors of diagnosing preterm birth. Cervical length measurement by transvaginal scanning (TVS) is shown to be more sensitive than the ‘traditional vaginal’ examination in the prediction of preterm birth. The most common cut off for cervical length is between 15 and 30 mm. A cut-off that is close to 30 mm has a high sensitivity with high false-positives in contrast to a cut-off close to 15 mm, which has lower sensitivity with less false-positives. The sensitivities of 81-100% with positive predictive values of 20-64% with the cervical length of 30-32 mm, are reported in a review that included 20 studies of cervical length measurement by transvaginal or transperineal ultrasonography in women admitted in threatened preterm labour3.

Cervical length measurement is superior to the fibronectin test in certain circumstances including ruptured membranes, vaginal bleeding, after vaginal examination and recent intercourse. A study using fibronectin concentration combined with cervical length measurement in patients with threatened preterm labour concluded that the fibronectin test does not improve the prediction when used together with cervical length measurements4. Transvaginal or transperineal scanning does however require a skilled clinician and a relatively high-resolution scanner.

TVS is shown to be well accepted by pregnant women5 and is safe as far as inoculation is concerned even with PPROM6;7. It is reproducible and inter-observer and intra-observer variability of TVS are both less than 10%8 compared with digital examination which has inter-observer variability of 52%9. TVS has an advantage over transabdominal sonographic evaluation as maternal habitus, position of the cervix and degree of bladder filling unlikely to affect visualisation of the cervix.

Transabdominal scanning provides essential information in preterm labour such as fetal presentation, as the incidence of malpresentation is higher in preterm deliveries, and estimated fetal weight – although there are limitations in its accuracy with advancing gestation or in the presence of anhydramnios and oligohyroamnios. The confirmation of fetal presentation is crucial as far as mode of delivery is concerned, and allows appropriate counselling for the mother. Estimated fetal weight particularly during a very preterm labour also allows paediatricians and obstetricians to counsel the parents accordingly and make joint plans as far as mode of delivery and neonatal resuscitation are concerned.

The absence of fetal breathing movements observed by ultrasound scanning is reported to be associated with preterm labour in patients admitted to labour ward with threatened preterm labour10. There are no randomised studies to suggest routine use in the diagnosis of preterm labour.

Unbooked pregnancies
A systematic review11 revealed that unbooked women had a significantly worse perinatal and maternal outcome. They were more likely to deliver preterm, have growth retarded babies and were at greater risk of a stillbirth or neonatal death. For women present in labour with no previous antenatal care, ultrasound scanning provides essential information including fetal presentation and placental position. Fetal biometry should be measured to help clinicians assess the correct gestational age if there is any doubt about this although sonographic biometry to date the pregnancy has limits in accuracy and may have a variation of 3 to 4 weeks especially at term. Similarly estimation of fetal weight may vary from actual fetal weight up to 10-15% more or less in the third trimester.

Induction of labour
Induction of labour occurs in 20% of pregnant women and 20% of these women end up with caesarean section12. Traditionally transvaginal examination for Bishops score is used to predict the success of induction to limit the number of failed inductions. Studies show that pre-induction cervical length measurement by TVS provides better prediction of successful induction of labour and mode of delivery than transvaginal examination which is subjective13 and has a poor predictive value for the outcome of induction14.

A recent meta-analysis included 20 trials with 3101 patients and showed that cervical length by TVS correlates with successful induction (LR 1.66; 95%CI, 1.20-2.31) and failed induction (LR 0.51; 95% CI, 0.39-0.67) but not with the mode of delivery15. The review also suggested that the assessment of cervical wedging may be a useful diagnostic test (an LR of a positive test result of 2.64 and an LR of a negative test result of 0.64) but needs further evaluation. Ultrasonographically determined fetal head position prior to induction is shown to be associated with mode of delivery. A significantly increased likelihood of Caesarean section is reported with occiput posterior position prior to induction16 however this concept remains controversial17.

Ultrasound scanning is used for labouring women for confirmation of presentation, placental position, visualisation of fetal heart beat and rate, fetal head position in relation to any fibroids, fetal head position during labour and prior to instrumental delivery. In the case of labouring breech malpresentation, it is used to identify the exact type of the breech, flexion of the fetal head, estimated fetal weight and the presence of nuchal cord to predict successful vaginal delivery. Other rare intrapartum uses have been reported for cervical effacement, station and engagement18-20.

A large prospective study involving 1633 women with a singleton pregnancy reported that clinical examination in the third trimester is not sensitive enough for detection of non-cephalic presentation with the values for sensitivity, specificity, positive predictive value and negative predictive value as 70% (95% CI 62% to 78%), 95% (94% to 96%), 55% and 97% respectively21. Therefore ultrasound scanning provides information on presentations particularly where maternal and fetal characteristics make the palpation difficult to allow appropriate management of malpresentation.

Ultrasonography has a paramount role in the evaluation of pregnant women presenting in labour with vaginal bleeding. A definitive diagnosis of low-lying placenta and vasa-praevia is achieved with ultrasound imaging. Low lying placenta may have not been previously detected or diagnosed but booked for a repeat scan at 36 week gestation repeat scan as recommended by the RCOG. Transvaginal ultrasound is safe and more accurate than transabdominal ultrasound in locating the placenta especially if it is positioned posteriorly. Clinical suspicion should be raised in any woman with vaginal bleeding and a high presenting part or malpresentation, regardless of previous imaging results as undiagnosed placenta praevia may result in serious maternal and neonatal morbidity at the time of delivery. However it might be difficult to diagnose “abruption” by ultrasound imaging unless the retroplacental haematoma is large enough in which case the clinical signs and symptoms should have alerted the clinicians anyway.

Intrapartum assessment of fetal head position is an integral part of routine monitoring in labour and is traditionally determined by transvaginal examination. Since the intrapartum ultrasonographic assessment of fetal head position is possible22 as shown figure below, studies unanimously confirmed the inaccuracies associated with transvaginal examination as shown in Table below. The largest study involving 496 patients showed that digital examination failed to define the fetal head position in 166 (33.5%) cases and in 330 cases where the position was determined, the findings of the digital and sonographic examinations were in agreement in only 163 (49.4%) of cases23. The rate of correct identification of the fetal position by digital examination is increased with cervical dilatation, from 20.5% at 3-4 cm to 44.2% at 8-10 cm and if the examination was carried out by an obstetrician rather than a midwife (50% versus 30%) and with the absence of caput (33% versus 25%).

Table 1: Studies on the accuracy of transvaginal examination using ultrasonography as a gold standard.
Author n Study Population Agreement N (%)
Sherer et al 2002 102 1st stage of labour 47%
Sherer et al 2002 112 2nd stage of labour 61%
Souka et al 2003 334 1st & 2nd stage of labour 46%
Akmal et al 2003 496 1st & 2nd stage of labour 49%
Dupuis el al 2005 110 2nd stage of labour 80%

It may be argued that the knowledge of the fetal head position in labour is less likely to affect the management of labour, provided cervical dilatation is satisfactory. There are however advantages of the determination of the fetal head position in early labour. Firstly, the vast majority of occiput posterior and transverse positions at delivery are a consequence of persistence of these positions during labour rather than malrotation24, therefore the determination of the occiput posterior position in the early stage of labour may alert clinicians to be more vigilant in the management of these cases. The use of syntocinon has been shown to reduce the incidence of occiput posterior positions at delivery (1% versus 5%) and can be administered early rather than later if there is any delay in labour progress and the need for one to one midwifery care can be planned. Secondly it could be used as a training tool after transvaginal examination to confirm the findings especially in prolonged labour where excessive caput, moulding and asynclitism are present. This is particularly important if the patient requires instrumental delivery, as an important determinant of successful and safe use of the vacuum and forceps is the correct determination of the fetal occiput position.

Anaesthesia related procedures
Regional anaesthesia in obstetrics is currently the gold standard for pain control including caesarean section and during labour in the majority of pregnant women. It is a blind procedure and is occasionally associated with failures and complications either due to difficult palpation or change in the anatomical landmarks. This may occur more commonly in obese patients. Ultrasound has recently been introduced into clinical practice to facilitate lumbar spinals and epidurals. One of the major advantages is the direct visualisation of nerves and anatomical structures including blood vessels, muscles and bones before the puncture resulting in a harmless and effective technique. A second advantage is the visualization of local anaesthetic entry during the procedure, which can alert the Anaesthetist about the need to immediately modify the technique or reposition, so as to improve the quality of the technique and distribution of the anaesthetic.

A recent review25 reported the advantages with the ultrasound-guided regional anaesthesia such as avoidance of side-effects including intra-neuronal and intravascular injection of local anaesthetic, avoidance of painful muscle injection, reduction of the dose of local anaesthetic, faster sensory onset time and longer duration of blocks.

The National Institute of Clinical Excellence (NICE) issued guidelines suggesting that ultrasound guidance should be used for all elective and emergency central venous catheterisations in order to minimise complications associated with central line placement. This statement involves the obstetric population with severe pre-eclempsia, major post-partum haemorrhage or maternal disease requiring central venous catheterisations. The use of ultrasound scanning amongst anaesthetists on labour ward is already on the increase in view of increasing obesity in the obstetric population which makes anaesthesia related procedures more difficult.

Figure: Occiput transverse, posterior and anterior positions detected in labour by ultrasound starting from left hand side

Caesarean section
In any pregnancies with malpresentation booked for elective/emergency caesarean section, ultrasound scanning should be performed to try to establish the cause of underlying problem and confirm that spontaneous version did not occur. In particular, placenta praevia and fibroids must be excluded and considered prior, rather than during, the operation. Visualisation of the lower segment and surrounding structures including the exact position of any fibroids, the edge of the placenta, the cord insertion and exact fetal presentation including the spine position in the case of transverse lie all allow better planning.

The presence of appropriately experienced staff during operation and appropriate level of preparation for the procedure such as need for cross-matched blood can be planned. It also provides better counselling for the parents before the procedure.

Most patients would have had an antenatal ultrasound scan, but the surgeons can plan a strategy for obtaining a safe access and delivery of the fetus by scanning just before surgery.

Amniotic fluid volume
There is controversy regarding intrapartum measurement of amniotic fluid volume as a predictor of adverse neonatal outcome. A meta-analysis with 18 studies included 10551 patients concluded that an intrapartum or antepartum amniotic fluid index of <5.0 cm is associated with an increased risk of caesarean section delivery for fetal distress (pooled RR, 1.7; 95% CI 1.1-2.6) and an Apgar score <7 at 5 minutes (pooled RR, 1.8; 95% CI, 1.2-2.7)26. It has also been suggested that the use of the single deepest pocket should be used rather than amniotic fluid index (AFI) which identifies a significantly greater number of women as having oligohydramnios but without any difference in perinatal outcomes and therefore such diagnosis may result in unnecessary interventions.

Amniotic fluid volume by ultrasound scan may be used as an adjunct to the diagnosis of spontaneous rupture of membranes (SROM) particularly before term. Speculum examination is usually diagnostic for SROM but in cases where liquor is absent on speculum examination and a good history of SROM, measurement of amniotic fluid volume by ultrasound may help the clinicians in the diagnosis and may modify their management of individual cases.

Post partum period
Ultrasound scanning is also used in the post-partum period for various reasons. First of all, it is used as a bedside rapid diagnostic tool in patients to rule out any intra-abdominal collection or intra-uterine collection/retained products. Ultrasound diagnosis of retained products of conception remains controversial in the management of secondary post-partum haemorrhage which affects 1-2% of obstetric population. It was shown that more than half of postpartum women admitted to hospital with bleeding undergo uterine surgical evacuation for the possible diagnosis of retained placental tissue although histological confirmation of residual placental tissue is obtained in only one third of cases.

In a recent review27, secondary postpartum haemorrhage, uterine curettage was performed in 63% of cases with high complication rates including uterine perforation in 3% and hysterectomy in about 1% of patients. Studies have concentrated on the ultrasonographic features of intrauterine retained products to improve the positive predictive value in detecting retained placental tissue by ultrasound scan and it still remains a challenge and more research is required. However once the decision is made for surgical removal of possible placental tissue, it is recommended that the procedure should be carried out under ultrasound guidance to reduce the complication rates associated with the surgery.

There are also studies reporting the safe and effective insertion of IUCD or IUS after the immediate delivery of placenta under ultrasound guidance without any increase in intrauterine infection and expulsion of the contraceptive device.

External cephalic version
ECV is the first line management of breech presentations if there is no contraindication to the procedure. The Royal College of Obstetricians and Gynaecologists (RCOG) recommended that ECV should be performed where ultrasound scanning is available for fetal heart rate visualisation and also stated that experience with ultrasound is essential. Ultrasound provides information on the type of the breech, head extension; liquor volume and exclusion of growth retarded fetuses which all influence either the performance or the success of the procedure.

Fetal well-being
Admission to labour ward for reduced fetal movements is not uncommon. The routine practice is to exclude intrauterine growth retardation (IUGR) or intrauterine death. Ultrasound scanning is used for visualisation of fetal heat beat and fetal movements and growth scan including amniotic fluid volume and Doppler studies if there is any suggestion of IUGR.

The readily available use of ultrasound scan on labour ward allows rapid diagnosis and prevents unnecessary delays, it reduces medico-legal litigations and increases both patient and clinician satisfaction. Currently, many patients admitted to the labour ward who require ultrasound scanning, are referred to the Ultrasound department.

This may be due to several factors: firstly in most units, the quality of scanner on labour ward is poor and does not show the necessary detail; and secondly, there are inadequate numbers of trained clinicians.

Adequate financial and training resources should be made available for the full use of ultrasound scanning on labour wards in order to improve efficiency and safety and to promote research.


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